A large number of mating components such as gears are used in a transmission mechanism of a vehicle configured to transmit a torque from a prime mover such as an internal combustion engine to a wheel. For example, a geared transmission is formed by a plurality of gear pairs and planetary gear mechanisms, and the geared transmission thus structured is configured to carry out a speed change operation by changing a power transmission route using an engagement device such as a clutch. Meanwhile, in a hybrid vehicle, an engine and an electric motor are connected with a planetary gear mechanism adapted to perform a differential action, and a rotational speed of the engine is controlled by changing a rotational speed of a reaction element of the planetary gear mechanism using the electric motor.
However, in the vehicle, the engine is kept being rotated even when the vehicle is being stopped, and in this situation, the gears are being engaged in the transmission mechanism. In the mechanism thus configured to transmit the torque by engaging the gearing mechanisms, the gears are still allowed to rotate relatively in the amount of backlash even under the situation in which the torque is not applied thereto. Therefore, in case the engine of the stopping vehicle is kept driven but the torque is not applied to the transmission mechanism, the gears of the transmission mechanism are vibrated and hit against another gear meshing therewith. Consequently, a rattling noise occurs in the transmission mechanism.
For example, Japanese Patent Laid-Open No. 8-145075 discloses a gear transmission device, in which a synchronizer ring used in a manual transmission is contacted frictionally with a driven gear to which torque is not being applied by moving the synchronizer ring in an axial direction. Under the neutral stage, a rotational speed of an input shaft becomes larger than that of an output shaft. Therefore, according to the teachings of Japanese Patent Laid-Open No. 8-145075, a predetermined rotary member on the output shaft is pushed in the axial direction utilizing such a speed difference, and the synchronizer ring is pushed by the predetermined rotary member to be contacted frictionally with the driven gear to which the torque is not being applied. In this situation, a braking torque is slightly applied to the driven gear. For this reason, vibrations and rattling of the driven gear can be reduced.
Thus, the gear transmission device taught by Japanese Patent Laid-Open No. 8-145075 is configured to utilize the frictional force of the synchronizer ring. Therefore, the gear transmission device taught by Japanese Patent Laid-Open No. 8-145075 is capable of reducing the rattling of the manual transmission, however, the gear transmission device taught by Japanese Patent Laid-Open No. 8-145075 cannot be applied to a vehicle which is not provided with a manual transmission. As described, in the known hybrid vehicle, the (internal combustion) engine and the electric motor are connected with the differential mechanism comprising three rotary elements such as a planetary gear mechanism, and an output element of the differential mechanism is connected with the output member through a gear to output the torque therefrom to the driving wheel. According to the hybrid vehicle thus structured, the neutral stage is established by idling the electric motor, and under the neutral stage thus established, the torque is not applied to the output member and the drive wheel. That is, in the hybrid vehicle, the engine is connected with the output member or the drive wheel in a manner to transmit the torque thereto even under the neutral stage as in the case of running the vehicle. In other words, under the neutral stage of the hybrid vehicle, the electric motor is idled and the torque thereof will not be applied to the output member and the drive wheel. This means that a power transmitting condition of a power transmission route from the engine to the drive wheel will not be changed in both drive stage and neutral stage. Therefore, in the hybrid vehicle, the gears on the power transmission route are vibrated by a fluctuation in the engine torque, and this makes a rattling noise. As described, the aforementioned synchronizer ring cannot be applied to a drive unit of the hybrid vehicle, therefore, the teachings of Japanese Patent Laid-Open No. 8-145075 cannot be applied to the hybrid vehicle to reduce vibrations and rattling of the gears.